1. Field of the Invention
The present invention relates to a data write method of an MRAM (Magnetic Random Access Memory) which uses, as a memory cell, a structure to store “1” or “0” information by using the magnetoresistive effect.
2. Description of the Related Art
In recent years, MRAMs (Magnetic Random Access Memories) using the tunneling magnetoresistive (to be referred to as TMR hereinafter) effect have been proposed as information memory elements (e.g., Roy Scheuerlein, et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC 2000 Technical Digest, p. 128).
An MTJ (Magnetic Tunnel Junction) element which exhibits TMR has an insulating film sandwiched between two ferromagnetic thin films. The MTJ element can create two states, i.e., a parallel state in which the magnetization directions of the upper and lower ferromagnetic materials are parallel to each other and an anti-parallel state in which the ferromagnetic materials are anti-parallel.
When the magnetization directions of the upper and lower ferromagnetic materials are parallel, the tunnel resistance of a current which flows to the thin insulating film sandwiched between the ferromagnetic materials is the lowest. This state is defined as, e.g., “1”. On the other hand, when the magnetization directions of the upper and lower ferromagnetic materials are anti-parallel, the tunnel resistance is the highest. This state is defined as, e.g., “0”. In this way, “1” or “0” information can be stored in the MTJ element.
A write in the MTJ element is executed by supplying a current to two write wirings perpendicular to each other and causing magnetization reversal in an MTJ element arranged at the intersection between the write wirings by generated magnetic fields. According to a coherent reversal model by Stoner-Wohlfarth, the energy necessary for causing magnetization reversal in the ferromagnetic material of the recording layer of the MTJ element is the lowest when a magnetic field is applied in a direction having an angle of 45° or 135° with respect to the direction of axis of easy magnetization of the recording layer. Hence, when the write wirings are arranged respectively in directions parallel and perpendicular to the direction of axis of easy magnetization, and a current is supplied to them, magnetization reversal can be caused in only the MTJ element located at the intersection between them. When MTJ elements are arrayed in a matrix on a two-dimensional plane, and write wirings are perpendicularly arranged in the row and column directions so that they pass on the upper and lower sides of the MTJ elements, a memory cell array can be formed.
However, if the coercively varies between the MTJ elements, or Neel coupling between the recording layer and the fixed (pinned) layer, field leakage from the fixed layer, or a deviation between the direction of magnetization vectors of the recording layer and fixed layer (except 0° and 180°) is present, the magnetic field necessary for magnetization reversal to the “0” state and that necessary for magnetization reversal to the “1” state may be different. In this case, when data is to be written in an MTJ element which requires a large write current for magnetization reversal, and an MTJ element capable of causing magnetization reversal only by the generated magnetic filed by the write current shares the wiring, a write error occurs, resulting in a decrease in reliability as the memory.
To the contrary, in the toggle write scheme described in U.S. Pat. No. 6,545,906, since the operation margin to the write error is large, the reliability as the memory can be increased. In this toggle write scheme, the recording layer of the MTJ element is designed such that, e.g., two ferromagnetic metal layers sandwich a paramagnetic metal layer to cause anti-ferromagnetic coupling. The composite magnetic moment of the recording layer is almost zero because the magnetic moments of the two ferromagnetic metal layers cancel each other.
The toggle write scheme is described in detail in U.S. Pat. No. 6,545,906. This will briefly be described. The composite magnetic moment of the recording layer, which appears due to the current magnetic fields by the write wirings, is rotated to reverse the magnetizations of two ferromagnetic metal layers of the recording layer. The reason why this scheme is tolerant to a write error to the half-selected MTJ applied the magnetic filed generated by only one current through one wiring will briefly be described. The composite magnetic moment of the recording layer, which appears due to the current magnetic fields by the write wirings, cannot be rotated by one directional magnetic field generated by one current through one wiring. When the write is ended, and no current is supplied longer, the applied magnetic field disappears, and the former state is inevitably restored by the anti-ferromagnetic coupling of two ferromagnetic layers of the recording layer.
In the example shown in U.S. Pat. No. 6,545,906, the direction of axis of easy magnetization of the ferromagnetic layer and anti-ferromagnetic layer of the MTJ elements has an angle of ±45° or ±135° with respect to the running direction of the write wiring. A uniform magnetic field is applied to the recording layer of the MTJ element. To do this, when the direction of thickness of the recording layer is defined as the Z-axis, the field component in the Z-axis direction must be almost zero. In addition, the applied magnetic field applied to the end portion of the recording layer the that applied to the central portion must have the same direction. When these conditions are taken into consideration, the wiring width must be larger than the easy- and hard-axis lengths of the MTJ element. According to Ampere's law, the magnitude of a magnetic field generated by a current becomes larger as the path of the magnetic field vector shortens. Hence, when the wiring width is large, the generated magnetic field decreases, and the current-field conversion efficiency becomes low. The current-field conversion efficiency may be increased by forming a flux keeper layer around the wiring. In this case, since it is difficult to form the flux keeper layer on the side of the wiring facing the MTJ element, the distance from the flux keeper layer to the recording layer is long. Hence, the current-field conversion efficiency decreases compared to a general structure in which the easy-axis direction is parallel or perpendicular to the write wiring.